In environmental processes, the contribution of macromolecular or colloidal metal complexes to the overall diffusion flux of metal may be of major importance. For labile complexes, the computation of such flux is based on the concept of average diffusion coefficient, which has only been checked systematically with small size ligands and in aqueous solution. This concept is checked here, both in aqueous solution and in agarose hydrogel, with large size macromolecules: dextrans of molar masses 10 000, 40 000 and 464 000, modified to introduce fluorescent or complexing (aspartate) moieties in their structure have been used. Their diffusion properties in water and agarose gel are determined by fluorescence correlation spectroscopy (FCS). The complexing and diffusive properties of their Pb(II) complexes in water and agarose hydrogel are also determined by scanning stripping chronopotentiometry (SSCP). It is shown that both in water and gel, the concept of average diffusion coefficient is applicable to compute the flux of these labile complexes at consuming interfaces. The results also show that the three ligands and their Pb-complexes diffuse as spherical coils, in water and in the gel, in accordance with other recent results. The implications of these results, in environmental applications, in particular in biouptake, and for the development of in situ sensors for environmental monitoring, are discussed, as well as the optimum conditions of voltammetric measurements with gel integrated microelectrode (GIME) in natural media containing colloidal complexes. In particular it is shown that, provided the gel is correctly preequilibrated with the test solution, and its thickness is larger than 150 μm, diffusion/reaction in the test solution do not influence the voltammetric flux at microelectrodes in the gel. The same conclusion is applicable to microorganisms inside biofilms.